This invention relates to pigment dispersions and, more particularly, to stable, fluid, aqueous pigment dispersions for use in film coating tablets and the like.
Suspensions or dispersions of pigments are used for producing coating compositions for film coating pharmaceutical tablets, capsules and the like. The pigments utilized are generally F D & C or D & C lakes or combinations of such lakes with titanium dioxide or iron oxide. The pigment suspension or dispersion is combined with other components (e.g., alcohols, plasticizers and film coating resins) to produce a coating composition used in film coating tablets and the like.
The process of manufacturing food grade lakes terminates in a filter press from which a solid cake is obtained. This filter cake is a stiff material handled as a solid and generally comprises 80% water and 20% lake solids. The possibility of developing a pigment dispersion with lakes in water must overcome the strong attractive forces in the aluminum hydroxide substrate of the lake to tie up large amounts of water in a solid matrix.
It is highly desirable that pigment dispersions or suspensions marketed for use in making film coating compositions have a high concentration of pigment or lake in water (e.g., 25 to 40 wt. %). It is also desirable to provide pigment dispersions or suspensions which are stable and fluid for extended periods of time and in which the pigment particles resist settling.
In the past, efforts have been made to produce pigment dispersions or suspensions having a high concentration of pigment therein. Signorino, U.S. Pat. No. 3,981,984 discloses high concentration pigment suspensions for polymer film coating of tablets and the like which comprise a non-aqueous solvent such as ethanol, pigment particles dispersed in the solvent and an edible protective colloid such as hydroxypropyl cellulose coating the pigment particles. More recently, Den Boer et al., U.S. Pat. No. 4,652,313 describes an aqueous lake pigment suspension comprised of a pigment, a natural or synthetic gum, a salt of a dicarboxylic or tricarboxylic acid compound such as a salt of citric acid or fumaric acid for lowering viscosity, and water. Based upon the data presented, the suspensions disclosed in U.S. Pat. No. 4,652,313 do not appear to provide the desired stability over extended periods of time and empirical data is required to formulate dispersions of various lake pigments with differences in formulations being required for different batches of the same lake.
There is a continuing need, therefore, for fluid, aqueous pigment dispersions which avoid the use of objectionable organic solvents, contain high concentrations of pigment and remain stable and useful for extended periods of time.